1. Field of the Invention
The present invention generally relates to a fixer, an image forming apparatus, such as a copier, a printer, a facsimile machine, a multifunction machine, etc., including the same, and a fixing method, and more particularly, to an electromagnetic induction heating fixer, an image forming apparatus including the same, and a fixing method using the same.
2. Discussion of the Background Art
In general, an electrophotographic image forming apparatus, such as a copier, a printer, a facsimile machine, and a multifunction machine including at least two of those functions, forms an electrostatic latent image on an image carrier, develops the latent image with developer such as toner, and transfers the developed image from the image carrier onto a sheet of recording media, such as paper, overhead projector (OHP) film, and the like, after which, the developed image (toner image) is fixed on the sheet.
A fixer is a mechanism that typically includes a fixing member such as a fixing roller and a pressure roller that presses against the fixing roller. The fixing member is heated by a heat source, typically but not necessarily internal to the fixing member, and the fixing member and the pressure roller together sandwich the sheet between them to form a fixing nip where the image formed on the sheet is fixed on the sheet with heat and pressure. This method is hereinafter referred to as the heating-roller fixing method.
Recently, various approaches described below have been tried to reduce both warm-up time and energy consumption of fixers. For example, one known fixer uses a fixing member such as an endless belt or film whose heat capacity is relatively small. Separately, an electromagnetic induction-heating fixing method has been proposed.
An electromagnetic induction-heating fixer generally includes an excitation coil through which a high-frequency electrical current is passed so as to generate a magnetic flux, and a magnetic core for guiding the magnetic flux to a roller-shaped or belt-shaped heat generator efficiently. A fixing nip can be formed by the heat generator and a pressure roller that presses against the heat generator directly or via a fixing member. When the pressure roller presses against the heat generator directly, the heat generator serves as a fixing member.
The magnetic flux causes an eddy current in the heat generator, and thus the heat generator is heated inductively. In this configuration, the heat generator can be promptly heated because the heat generator itself can generate heat, eliminating preheating that is required in the heating-roller fixing method. Thus, the electromagnetic induction-heating fixing method is advantageous in that both warm-up time and energy consumption can be reduced.
However, the electromagnetic induction-heating fixing method still has a problem described below in detail.
Generally, the image forming apparatus can accommodate a variety of different sheet sizes. When sheets whose width, that is, length in a direction perpendicular to a direction in which the sheets are transported (hereinafter “sheet width direction”), is relatively small pass through the fixing nip continuously, lateral end portions of the heat generator (or the fixing member) where the sheets do not pass (hereinafter also “non-sheet area”) tend to overheat. This is because heat of a portion of the heat generator where the sheet passes (hereinafter “center portion” or “sheet area”) is drawn by the sheet and heat of the lateral end portions is not lost.
Therefore, temperature can rise excessively in the end portions of the heat generator, degrading or even damaging the heat generator. This phenomenon is hereinafter referred to as excessive heating at end portions.
Further, when a sheet whose width is larger than that of the small sheets passes the fixing nip after the small sheets have passes the fixing nip continuously for some time, toner in a resulting image will be partly absent in portions of the sheet that pass the overheated end portions of the heat generator, which is a phenomenon called hot offset. Hot offset occurs because, when toner is heated excessively, cohesion among toner particles is lower than adhesion between the toner particles and the fixing member, thereby, causing toner layers to separate.
In view of the foregoing, one known technique suggests using sub-induction coils or demagnetization coils for counteracting the magnetic flux generated by a main induction coil or excitation coil. The demagnetization coils are respectively provided in end portions of the heat generator except an area to be covered by a sheet whose width is smallest (hereinafter “smallest sheet”) among multiple different sheet sizes that the image forming apparatus can accommodate. When the smallest sheet passes the fixing nip, the demagnetization coils are energized so as to counteract the excitation magnetic flux that is to act on the non-sheet area, restricting temperature rise at the end portions. By contrast, when a sheet whose width is larger than the smallest width passes the fixing nip, power is not supplied to the demagnetization coils, and thus the excitation magnetic flux acts on whole the width of the heat generator, heating whole the heat generator.
In this configuration, although excessive heating at end portions (non-sheet area) can be restricted, if the demagnetization coils are relatively far from the area to be covered by the smallest sheet (hereinafter “smallest-sheet area”) in the sheet width direction, temperature can excessively rise in portions between the smallest-sheet area and the portions corresponding to the demagnetization coils, degrading those portions. By contrast, if the demagnetization coils overlap the smallest-sheet area in the sheet width direction, the amount of heat will be insufficient in end portions of the smallest-sheet area due to a sudden decrease in the magnetic flux, making the temperature in the smallest-sheet area uneven. Thereby, fixing failure, offset, and/or unevenness in gloss can be caused in a fixed image.
In view of the disadvantage described above, the following techniques have been proposed.
One known technique suggests using a demagnetization coil looped into a particular shape so as to prevent both excessive heating at end portions and unevenness in temperature in the sheet area in an axial direction (sheet width direction) of the fixing member. The demagnetization coil has a curved end portion and is disposed so that the curved end portion overlaps an end portion of the sheet in the sheet width direction. More specifically, because a demagnetization effect of the curved end portion is lower than that of a portion extending in the axial direction, by disposing the curved end portion to overlap the end portion of the sheet in the sheet width direction, differences in temperature between the center portion and the end portions in the sheet width direction of the sheet area can be reduced.
Further, another known technique suggests using divided multiple demagnetization coils each having a particular shape and disposing them in accordance with multiple different sheet sizes. In this technique, the multiple demagnetization coils can be energized separately in accordance with each sheet size.
However, an additional complication in this regard is the relation between arrangement of the magnetic cores and density of the magnetic flux. More specifically, in the known fixers described above having the demagnetization coil whose end portion in the sheet width direction is curved and multiple magnetic cores are arranged in the sheet width direction, the magnetic cores cannot be continuous in the sheet width direction even if the multiple magnetic cores are respectively disposed in areas enclosed by both the excitation coil and the demagnetization coil and an area enclosed by only the excitation coil. Where the center core is partly absent, the magnetic flux density can decrease in a portion of the heat generator facing such a portion, and thus the temperature thereof will drop.
Therefore, there is a need to prevent a drop in temperature of the heat generator as well as excessive heating at the end portions thereof in the sheet width direction, which the known methods fail to do.